[PATCH] D143673: [lld][RISCV] Implement GP relaxation for R_RISCV_HI20/R_RISCV_LO12_I/R_RISCV_LO12_S.

[Fangrui Song via Phabricator via llvm-commits]

MaskRay added inline comments.


================
Comment at: lld/test/ELF/riscv-relax-hi20-lo12.s:6
+# RUN: llvm-mc -filetype=obj -triple=riscv32-unknown-elf -mattr=+c,+relax %s -o %t.rv32c.o
+# RUN: llvm-mc -filetype=obj -triple=riscv64-unknown-elf -mattr=+c,+relax %s -o %t.rv64c.o
+
----------------
craig.topper wrote:
> MaskRay wrote:
> > craig.topper wrote:
> > > MaskRay wrote:
> > > > We may need a test for `-pie`.
> > > -pie should use %pcrel relocations right?
> > We should have a meaningful diagnostic referencing a non-absolute symbol with an absolute relocation in -pie mode. It probably doesn't belong to this test.
> > 
> > In addition, it's valid to reference SHN_ABS symbols in `-pie` mode. We should test that SHN_ABS symbols are correctly handled. Whether it's handled doesn't matter since such cases are super rare.
> How do I make an SHN_ABS symbol?
Something like

```
.globl abs
abs = 0x10000
abs_local = 0x1000
```


================
Comment at: llvm/include/llvm/BinaryFormat/ELFRelocs/RISCV.def:49
 ELF_RELOC(R_RISCV_RVC_LUI,           46)
+ELF_RELOC(R_RISCV_GPREL_I,           47)
+ELF_RELOC(R_RISCV_GPREL_S,           48)
----------------
craig.topper wrote:
> MaskRay wrote:
> > craig.topper wrote:
> > > craig.topper wrote:
> > > > craig.topper wrote:
> > > > > jrtc27 wrote:
> > > > > > craig.topper wrote:
> > > > > > > jrtc27 wrote:
> > > > > > > > These don't exist, they were removed as they were solely internal to binutils, and their encodings are reserved for future standard use
> > > > > > > Is binutils only going to be updated to use different values when a new standard use is introduced?
> > > > > > > 
> > > > > > > Is there a safe number to use for LLD insternal use?
> > > > > > So long they're not permitted in inputs it's "fine" as it won't misinterpret future files, but using ELF_RELOC here means all the other tools will think this is a real relocation (e.g. llvm-readelf will decode it). Can you not just use R_RISCV_RELAX like for TLS, though? It seems you're doing the exact same thing.
> > > > > In order to support .sbss, I believe I need to create a relocation to communicate from the instruction relaxation phase to the `relax` function which runs later. When I tried to rewrite .sbss globals in `relaxHi20Lo12` I got the wrong addresses. My thinking was that I needed the .text section to shrink and the layout of later sections to adjust before I could do the instruction rewriting.
> > > > Since R_RISCV_GPREL_I/R_RISCV_GPREL_S are in the normal relocation list in binutils, doesn't that mean binutils readelf will also think they are real relocations?
> > > @jrtc27 what should I do here? As I said, I think GNU readelf also thinks they are real relocations.
> > They are ABI-removed relocation types only for GNU ld's internal use. Since these relocation types are not allowed in input files, we can define the constants only in an lld/ELF file.
> Is there an existing file for that? And are there different values I should use?
No existing file use internal relocation type this way. But you can just define two constants in `lld/ELF/Arch/RISCV.cpp`. (`R_RISCV_*` will be poor names since they aren't official relocation type names.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D143673/new/

https://reviews.llvm.org/D143673